Powder metallurgy is a technique in which powder, which is usually metal, is compacted in a mold into a powder green compact and the green compact is sintered at temperatures slightly lower than the melting temperature of the whole compact, thereby producing a sintered compact, that is, a product. The technique can manufacture a large number of products of identical shape inexpensively, and is therefore widely used in the production of automobile parts, machinery parts, magnetic materials and cutting tools.
However, it has been difficult to produce products with complicated shapes by conventional powder metallurgy processes. The powder green compacts prepared by the conventional processes tend to have internal pores and consequent low mechanical strength, and thus intricately-shaped green compacts are easily broken. Because of the low mechanical strength of the green compacts, the mechanical processing of compacts with a simple shape into a complicated shape is difficult.
In relation to the manufacturing of compacts, high-speed centrifugal compaction processes have been studied. In the high-speed centrifugal compaction process, a material powder is homogeneously dispersed in a solvent to give a slip, which is poured into a mold and subjected to centrifugal force on a centrifuge whereby the material powder is sedimented on the bottom of the mold and is shaped to the mold configuration. According to the high-speed centrifugal compaction processes, a fine material powder can be compacted highly densely and uniformly and the obtainable compacts are free of internal pores. Because the high-speed centrifugal compaction processes allow for high mechanical strength of compacts, the compacts can be formed into a relatively complicated configuration. For example, JP-A-2005-48230 (Patent Document 1) describes that a core for giving a desired shape to a compact is inserted in a mold, and a material powder is compacted in such a mold whereby it can be easily formed into a relatively intricate shape. According to this technique, the material powder is settled down by the centrifugal force while the core inserted in the mold undergoes only the substantially isotropic static water pressure, and thus a compact can be formed containing the core without collapse even if an intricately-shaped core is used.
When the compacts are designed with a highly complicated shape, however, even the high-speed centrifugal compaction processes result in cracks or the like at heavily irregular portions. Even the process disclosed in JP-A-2005-48230 cannot prevent the occurrence of cracks or the like. Further, damages are often caused when the compacts prepared by the high-speed centrifugal compaction processes are machine processed into a highly complicated shape. Therefore, the high-speed centrifugal compaction processes are not suited for the production of highly intricate compacts such as precursors of diesel engine fuel injection nozzles. As a result, it has been difficult to produce sintered compacts having a very complicated shape by the powder metallurgy processes.    Patent Document 1: JP-A-2005-48230